As the demands for higher and higher data rates from a mobile user in a cellular communication system increase, on one hand, a higher peak data rate is expected for an individual user at a time, and on the other hand, those advanced techniques to increase the spectrum efficiency are of more interests. Due to the limitation of available radio spectrum, high data rates would be shared by as many mobile users as possible.
One of the most promising techniques is MIMO, which enables not only multiple data streams in the same bandwidth for an individual user, as illustrated in FIG. 1, but also multiple data streams from different users in the same bandwidth, as shown in FIG. 2. Even further, two users with different air interfaces could transmit radio signals in the same bandwidth.
In the above mentioned cases, the crossing data streams from the same user or different users interfere with each other, thus in-band interferences exist from prospective of each data stream. That is, when demodulating and decoding stream 1, there is signal component from stream 2, and vice versa. This signal component is actually interference besides the background noise. Therefore, at the receiver side some special advanced techniques must be involved to relieve or cancel the cross interferences among data. Sequential interference cancellation (SIC) is one of the techniques for relieving or cancelling the cross interferences, as shown in FIG. 3.
The idea of an SIC receiver is to take the advantage of the case where one data can be decoded correctly in terms of CRC checksum. Of all the crossing data streams, if one CRC is observed, then the data stream is re-encoded and re-modulated, then re-generated together with the channel estimates. The regenerated signal is then subtracted from the total received signal resulting in a residue signal. Finally a second attempt to demodulate and decode is performed on the residue signal for the rest of the crossing data streams whose CRC checksum is not OK.
FIG. 4 shows another example of a so-called turbo-SIC receiver. The turbo-SIC receiver even takes more advantage of the CRC-OK data stream by taking out the Log Likelihood Ratio (LLR) values after decoding and performing soft modulation to reconstruct the signal constellation estimates then regenerating the signal as previous example. LLR values after decoding are regarded to be more reliable thus good for the signal regeneration. Some substantiate performance gain is expected from Turbo-SIC at the cost of complexity.
Both SIC and Turbo-SIC receivers use the CRC checksum as the only criteria for the decision whether the concerned signal should be regenerated or not. Nature CRC checksum has zero tolerance to a single error bit, which means that the interference signal is able to be regenerated only when all of the decoded bits are correct, therefore with this post-decoding regeneration, the criteria is actually too high thus removes the possibility where most of the decoded bits are correct. The key point is to find an alternative criteria, which is not so strict as the CRC checksum.